The overall aim of this project is to better understand the role of impulsivity in drug addiction. Deficits in impulsivity are implicated in virtually all kinds of addictive behaviors and are related to risky sexual and injection practices, some of the primary vectors of HIV transmission. However, impulsivity is one of the most multifaceted constructs described in the literature that has a variety of trait-like personality and psychiatric dimensions, as well as performance-based neurobehavioral manifestations. Although many substance dependent individuals (SDIs) show impaired impulse control on performance-based neurobehavioral measures of impulsivity, certainly not all SDIs manifest such impairments. This suggests that individual differences in some additional risk factors such as the more temporally stable personality and psychiatric dimensions of impulsivity may increase vulnerability to performance-based neurobehavioral impulse control deficits in SDIs. Such performance-based deficits may also be a function of the unique pharmacological / toxicological properties of the specific drug of abuse. Therefore, one of the main goals of the project is to investigate how different conceptual approaches to impulsivity differ in their manifestations in SDIs and whether such differences are a function of the specific type of illicit drug used. An additional goal of the project is to examine which of the various dimensions of impulsivity are most closely related to high-risk sexual and injection risk behaviors and whether such high-risk behaviors differ as a function of the specific drug of abuse. The study has the following specific aims: (1) To investigate the associations of different classes of drugs (opiates and stimulants) and trait-like personality and psychiatric (ADHD and psychopathy) indices of impulsivity on performance on neurobehavioral measures of cognitive and motor impulsivity among groups of pure stimulant users, pure opiate users, and non-drug using controls. (2) To investigate the patterns of associations between individual differences in temporally stable trait-like personality and psychiatric indices of impulsivity with performance-based measures of motor and cognitive impulsivity. (3) To evaluate the associations between different trait- and performance-based manifestations of impulsivity with high risk behaviors that increase risk for HIV infection and transmission, such as sexual and injection risk practices among opiate and stimulant users. The research will be conducted in Bulgaria, where patterns of drug use are unique in that polysubstance dependence is uncommon, which will help us avoid the challenging methodological problem of polysubstance use among SDIs. Data from the proposed study will provide new knowledge on impulsivity in SDIs, on the specific effects of different classes of drugs of abuse on various dimensions of impulsivity, and bridge the gap on how individual differences in impulsivity might influence critical risky behaviors. Therefore, the study is expected to have important implications for the development of biological and psychological interventions for SDIs and for building sustainable research capacity in Bulgaria. RELEVANCE The project is expected to have important implications for the development of biological and psychological prevention and intervention strategies for substance dependent individuals. More specifically, the project will be particularly relevant for helping increase treatment efficacy by matching clients to treatment, based on individual differences in various dimensions of impulsivity, which could be addressed by various treatment modalities or techniques. The project could also have significant implications for HIV prevention efforts by informing harm-reduction prevention and intervention strategies aimed at reducing high-risk sexual and injection behaviors in drug users, who are some of the primary vectors for HIV transmission.